docs: def add_resource(self, resource):
          '''Perform an atomic prepend for a new resource'''
          resource.validate()
          self.update(__raw__={
              '$push': {
                  'resources': {
                      '$each': [resource.to_mongo()],
                      '$position': 0
                  }
              }
          })
          self.reload()
          post_save.send(self.__class__, document=self,
                         resource_added=resource.id)